Switched supply for operational amplifier

ABSTRACT

There is provided a circuit and method for providing a supply voltage to an operational amplifier. A switch has a plurality of inputs connected to a respective plurality of supply voltages. An output of the switch is connected to a supply voltage terminal of an operational amplifier. The input of the switch is selected in dependence of the voltage levels to which a signal is to be amplified

FIELD OF THE INVENTION

[0001] The present invention relates to a technique for providing supplyvoltages to a supply terminal of an operational amplifier.

BACKGROUND TO THE INVENTION

[0002] In certain applications the voltage levels to which anoperational amplifier is required to drive signals is variable. Thus,for example, the operational amplifier may at times be required to driveoutput signals to a voltage level of 15 volts, whilst at other times itmay only be necessary to drive output signals to a voltage level of 5volts. However in order to cover the full range of possible outputvoltage levels, the operational amplifier is required to be providedwith the voltage supply corresponding to the highest voltage level,namely in this example 15 volts.

[0003] It is therefore an object of the present invention to provide animproved technique for providing a supply voltage to a supply terminalof an operational amplifier.

SUMMARY OF THE INVENTION

[0004] In accordance with a first aspect of the present invention thereis provided a circuitry for providing a supply voltage to an operationalamplifier, comprising:

[0005] a switch having a plurality of inputs connected to a respectiveplurality of supply voltages, and an output connected to a supplyvoltage terminal of the operational amplifier, wherein the input of theswitch is selected in dependence on the voltage level to which a signalis to be amplified. Thus, advantageously the operational amplifier isonly provided with a supply voltage level corresponding to the highestvoltage level to which it requires to drive a signal, thereby savingpower consumption when only low voltage levels are needed.

[0006] A digital to analogue converter may receive digitised values, andgenerate a corresponding analogue signal for amplification by theoperational amplifier. The input of the switch may be selected independence on the digitised values. The input of the switch may beselected in dependence on either the peak digitised value of thedigitised values or the average digitised value of the digitised values.

[0007] The input of the switch may be selected by a control signalgenerated in dependence on the digitised values. The digitised valuesmay be stored in the digital to analogue converter.

[0008] Preferably the switch has a first and a second input connected toa respective first and second supply voltages, a first supply voltagebeing lower than the second supply voltage, wherein the input of theswitch is selected to be the second input if the voltage level to whichthe signal is to be amplified exceeds a predetermined level.

[0009] The circuitry for providing the supply voltage to the operationalamplifier may further comprise:

[0010] a further switch having a plurality of inputs connected to arespective plurality of further supply voltages, and an output connectedto a further supply voltage terminal of the operational amplifier,wherein the input of the further switch is selected in dependence on thevoltage level to which the signal is to amplified.

[0011] In a further aspect the present invention provides a method ofproviding a supply voltage to an operational amplifier, comprising thesteps of:

[0012] providing a plurality of supply voltages; selecting one of thesupply voltages in dependence on the voltage level to which a signal isto be amplified; and connecting the selected one of the plurality ofsupply voltages to a supply voltage terminal of the operationalamplifier.

[0013] The method may further comprise the step of converting digitisedvalues into an analogue signal for amplification by the operationalamplifier. The step of selecting one of the supply voltages may bedependent upon the digitised values. The step of selecting one of thesupply voltages may be dependent upon either the peak digitised value ofthe digitised values, or the average digitised value of the digitisedvalues.

[0014] The step of selecting one of the supply voltages may includegenerating a control signal in dependence of the digitised values.

[0015] Preferably there is provided a first and second supply voltage,the first supply voltage being lower that the second supply voltage,wherein the second supply voltage is selected if the voltage level towhich the signal is to amplified exceeds a predetermined level.

[0016] The method of providing the supply voltage to the operationalamplifier may further comprise the steps of:

[0017] providing a plurality of further supply voltages; selecting oneof the further supply voltages in dependence on the voltage level towhich a signal is to be amplified; and connecting the selected one ofthe plurality of further supply voltages to a further supply voltageterminal of the operational amplifier.

[0018] The circuitry for providing the supply voltage to the operationalamplifier, or the method for providing a supply voltage to theoperational amplifier, may be associated with an xDSL modem. A DSL is adigital subscriber line. The term “x” in front of DSL implies that theinvention is relevant to any type of digital subscriber line technology.For example, the invention is relevant to ADSL (asymmetric digitalsubscriber line) technology, the VDSL (very-high-data-rate digitalsubscriber line) technology, HDSL (high bit rate digital subscriberline) technology etc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention will now be described with regard to a non-limitingexample by way of reference to the accompanying drawings in which:

[0020]FIG. 1 illustrates a first embodiment of a circuitry for providinga supply voltage to an operational amplifier in accordance with thepresent invention; and

[0021]FIG. 2 illustrates a second embodiment of circuitry for providinga supply voltage to an operation amplifier in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The following example of the present invention is described withspecific reference to an application in an ADSL (asymmetric digitalsubscriber line) implementation. It will be apparent to the personskilled in the art that the present invention is generally applicable toany operational amplifier implementation.

[0023] Referring to FIG. 1, the operational amplifier block according tothe present invention as shown in this embodiment is generallydesignated by reference numeral 10. The block comprises an operationalamplifier 14 and a switch 34. For the purposes of describing the presentinvention in this example there is also provided a digital to analogueconverter 12 and a logic gate 16. The digital to analogue converter 12receives digital values on line 18, and generates an analogue signal onlines 20 and 22 to the positive and negative inputs of the operationalamplifier 14 as is well known in the art. The operational amplifier 14has a first supply voltage terminal 36 connected to a node 38 of theswitch 34. A second supply voltage terminal 24 of the operationalamplifier 14 is connected to ground. The operational amplifier 14generates amplified analogue signals on line 26. A switch 34 has twoadditional nodes 40 and 42. Node 40 is connected to a supply voltageV_(CCL) on line 30. Node 42 is connected to a supply voltage V_(CCH)which is connected to line 32. An output of a digital to analogueconverter 12 on line 28 is provided to logic gate 16 which in turnprovides a control signal on line 30 to the switch 34.

[0024] As can be illustrated by the arrows within the switch unit 34,the switch 34 is controllable to connect node 38 to either the node 40or 42. In this way the first supply terminal 36 of the operationalamplifier receives either the supply voltage V_(CCL) on line 30 or thesupply voltage of V_(CCH) on line 32.

[0025] The digitised values provided on line 18 to the digital toanalogue converter 12 are indicative of the voltage levels to which theanalogue signal on the output on line 26 of the operational amplifierare to be driven to. Thus, these values are indicative of the supplyvoltage level, either V_(CCL) or V_(CCH), which the first supplyterminal 36 of the operational amplifier should be connected to. Thusthe logic gate 16 will generate a signal on its output 30 in dependenceon the value of line 18 provided to the digital to analogue converter 12meeting certain predetermined conditions. Thus the logic gate 16 may beconfigured to detect a digitised value above a certain threshold value,and responsive thereto may set the signal on line 30 to switch to thehigher supply voltage V_(CCH).

[0026] Alternatively, rather than the logic gate 16 being configured toswitch to a high supply voltage responsive to a single peak beingdetected above a threshold level, the logic gate 16 may be adapted toaccumulate the digitised values and determine the average value thereof,and only response to the average value being above a peak is the controlsignal 30 set to switch to the higher supply voltage V_(CCH).

[0027] Referring to FIG. 2, there is illustrated a second exemplaryembodiment for controlling an operational amplifier according to thepresent invention. Like reference numerals are used to denote elementscorresponding to elements shown in FIG. 1.

[0028] Thus, as can be seen from FIG. 2, the operational amplifierblock, designated by reference numeral 11, is modified to include afurther switch 50 for controlling a supply voltage provided to thesecond supply voltage terminal 24 of the operational amplifier 14. Thesecond supply voltage terminal 24 of the operational amplifier 14 isconnected to a node 56 of the switch 50. The switch 50 additionallycomprises two nodes 52 and 54. The node 52 is connected to the supplyvoltage level V_(SSL) on line 58, and node 54 is connected to a supplyvoltage level V_(SSH) on line 60. In addition the switch 50 receives thecontrol signal on line 30 from the logic gate 16.

[0029] The operation of the general circuit of FIG. 2 is exactly thesame as FIG. 1, with the exception that the control signal 30 nowadditionally controls the supply voltage applied to the second supplyvoltage terminal 24 of the operational amplifier. Thus, by way ofexample, the voltage level of V_(CCL) may correspond to plus five volts,and the voltage level of V_(SSL) correspond to minus five volts. Innormal operation, the switches 34 and 50 may have a default setting inwhich the supply voltage terminal 36 is connected to the voltageV_(CCL), and the supply voltage terminal 24 is connected to the voltageV_(SSL.) In this example, it may be considered that the voltage levelV_(CCH) is plus fifteen volts and the voltage level V_(SSH). is minusfifteen volts. Responsive to the appropriate peak or average levelsbeing detected in the digitised signals by the logic gate 16, the line30 is set such that the switch 34 switches to connect the voltageV_(CCH) the supply voltage terminal 36, and the switch 50 switches toconnect the supply voltage terminal 24 to the supply voltage levelV_(SSH).

[0030] Thus it can be seen that with the use of the present inventionpower can be conserved by only applying the high voltage levels to theoperational amplifier when it is required to amplify a signal to ahigher voltage level.

What is claimed is:
 1. Circuitry for providing a supply voltage to anoperational amplifier, comprising: a switch having a plurality of inputsconnected to a respective plurality of supply voltages, and an outputconnected to a supply voltage terminal of the operation amplifier,wherein the input of the switch is selected in dependence on the voltagelevel to which a signal is to be amplified.
 2. Circuitry according toclaim 1 further comprising a digital to analogue converter for receivingdigitised values and for generating a corresponding analogue signal foramplification by the operational amplifier.
 3. Circuitry according toclaim 2 wherein the input of the switch is selected in dependence on thedigitised values.
 4. Circuitry according to claim 3 wherein the input ofthe switch is selected in dependence on the peak digitised value. 5.Circuitry according to claim 3 wherein the input of the switch isselected in dependence on the average digitised value.
 6. Circuitryaccording to claim 2 wherein the input of the switch is selected by acontrol signal generated in dependence on the digitised values. 7.Circuitry according to claim 4 wherein the digitised values are storedin the digital to analogue converter.
 8. Circuitry according to claim 1,wherein the switch has a first and a second input connected torespective first and second supply voltages, the first supply voltagebeing lower than the second supply voltage, wherein the input of theswitch is selected to be the second input if the voltage level to whichthe signal is to be amplified exceeds a predetermined level. 9.Circuitry for providing a supply voltage to an operation amplifieraccording to any claim 1, comprising: a further switch having aplurality of inputs connected to a respective plurality of furthersupply voltages, and an output connected to a further supply voltageterminal of the operational amplifier, wherein the input of the furtherswitch is selected in dependence on the voltage level to which thesignal is to be amplified.
 10. An xDSL modern including circuitryaccording to claim
 1. 11. A method of providing a supply voltage to anoperational amplifier, comprising the steps of: providing a plurality ofsupply voltages; selecting one of the supply voltages in dependence onthe voltage level to which a signal is to be amplified; and connectingthe selected one of the plurality of supply voltages to a supply voltageterminal of the operational amplifier.
 12. The method of claim 11further comprising the step of converting digitised values into ananalogue signal for amplification by the operational amplifier.
 13. Themethod of claim 12 wherein the step of selecting one of the supplyvoltage is dependent upon the digitised values.
 14. The method of claim13 wherein the step of selecting one of the supply voltages is dependentupon the peak digitised value.
 15. The method of claim 13 wherein thestep of selecting one of the supply voltages is dependent upon theaverage digitised value.
 16. The method of claim 12 wherein the step ofselecting includes generating a control signal in dependence on thedigitised values.
 17. The method according to claim 11 in which there isprovided a first and a second supply voltage, the first supply voltagebeing lower than the second supply voltage, wherein the second supplyvoltage is selected if the voltage level to which the signal is to beamplified exceeds a predetermined level.
 18. A method of providing asupply voltage to an operational amplifier according to claim 11comprising the steps of: providing a plurality of further supplyvoltages; selecting one of the further supply voltages in dependence onthe voltage level to which a signal is to be amplified; and connectingthe selected one of the plurality of further supply voltages to afurther supply voltage terminal of the operational amplifier.
 19. Amethod of providing a supply voltage to an operational amplifier of anxDSL modem according to claim 11.